Progressive burning propellants suitable for use as the propellant powder for selected ammunition are generally prepared by adding an organic material, which is a gelatinizing agent for the nitrocellulose component of the propellant, to the propellant granules and tumbling said mixture in a rotating barrel which is heated above the melting point of the added organic material. The organic material becomes fluid upon heating and coats the powder. During the coating operation the mixture is usually wet with water. Water may be added directly to the rotating barrel or may be supplied in the form of condensed steam during heat-up of the organic coating material. This coating technique is satisfactory for use with propellants which are of the single base or double base type, i.e., propellants consisting essentially of nitrocellulose and an explosive plasticizer such as nitroglycerin. This coating technique is not satisfactory for use with propellant powder which contains in addition to nitrocellulose and explosive plasticizer, water-soluble components, e.g., nitroguanidine, or other water-soluble materials. When propellants containing nitroguanidine are coated in the presence of water or organic liquids water-soluble components such as nitroguanidine are leached out of the propellant composition and the burning characteristics of the propellant powder are adversely affected.
U.S. Pat. No. 2,992,911 discloses a process for coating the surface of nitroguanidine containing propellant powder for the purpose of preparing progressive burning propellant compositions. In accordance with the teachings of this patent, the surface of propellant powder comprising nitrocellulose, nitroglycerin and nitroguanidine is coated with a burning rate deterrent composition, said process comprising treating the powder surface with symmetrical diethyl diphenyl urea in a quantity sufficient to form a chemical complex in situ by interaction of the diethyl diphenyl urea with said nitroguanidine in a zone of inwardly diminishing concentration in the neighborhood of the surface only of said propellant powder. In practice, the diethyl diphenyl urea is dissolved in a nonsolvent for nitrocellulose such as ethyl alcohol and the resulting solution is sprayed on the powder in a rotating pan or barrel. The resulting surface modified propellant powder has progressive burning characteristics. The ballistic properties of such propellant powder are said to be substantially unaltered after storage at 65.degree. C. for six months.
U.S. Pat. No. 3,743,554 discloses a method of deterrent coating of smokeless powder employing certain linear polyesters as the burning rate deterrent medium. In the process described in this patent, smokeless powder is agitated in a mixing tank in the presence of water containing approximately 2% of the polyester deterrent coating material. The deterrent coating material is added as a solution in methylene chloride solvent. After addition of the deterrent coating material to the mixing tank the resulting suspension is heated to 85.degree.-90.degree. C. for at least 30 minutes. At the end of the thirty minute period the coating of the smokeless powder is complete. The linear polyester burning rate deterrent materials employed in U.S. Pat. No. 3,743,554 are formed by reaction of dihydric alcohol components such as ethylene glycol, polyethylene glycol, propylene glycol, propylene polypropylene glycol and neopentyl glycol, and the like with dibasic acid components such as adipic acid, azaleic acid, phthalic acid and sebacic acid and the like. The polyester materials have a weight average molecular weight of from about 1,500 to about 30,000 and a melting point not exceeding about 190.degree. F.
German OLS No. 2,060,052, published June 8, 1972, discloses a process for preparing progressive burning propellant powder by surface treating powder strands with desensitizing materials prior to cutting to final dimensions so that only external surfaces of strands are desensitized. In the process disclosed, solvent containing powder strands are passed vertically through compartments in which surface treatment is carried out by a spraying operation. The strands may be partially dried prior to entrance into the spraying compartments. It is also suggested that surface treatment can be conducted by drawing elongated powder strands through appropriate solutions.